Support custom json decoder and encoder when dealing with JSON/JSONB columns

At the moment, every-time a Postgres json value is decoded, a new `Foundation.JSONDecoder` is created on the spot for the decoding. The same thing is done for encoding json columns. 

For performance (notably on Linux) and floating point encoding accuracy reasons, we should be able to use custom, i.e non-Foundation, json encoders and decoders when dealing with json and jsonb columns. 